1. Field
The present invention relates generally to data communication, and more specifically to a multiple-access hybrid OFDM-CDMA communication system.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users (sequentially or simultaneously) by sharing the available system resources (e.g., bandwidth and transmit power). Such systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), some other multiple access technique, or a combination thereof. CDMA systems may provide certain advantages over other types of system, including increased system capacity. CDMA systems may also be designed to implement known CDMA standards such as IS-95, cdma2000, IS-856, W-CDMA, and others.
An orthogonal frequency division modulation (OFDM) system effectively partitions the system bandwidth into a number of (M) sub-bands (or frequency bins or sub-channels). At each time interval that may be dependent on the bandwidth of each sub-band, a modulation symbol may be transmitted on each of the M sub-bands.
In a direct sequence (DS) CDMA system, a narrowband signal is spread over the entire system bandwidth in the time domain with a spreading sequence. Example DS-CDMA systems include those that conform to IS-95, cdma2000, and W-CDMA standards. The spreading sequence may be a pseudo-random noise (PN) sequence (e.g., for IS-95 and cdma2000) or a scrambling sequence (e.g., for W-CDMA). A DS-CDMA system provides certain advantages such as ease of supporting multiple access, narrow-band rejection, and so on.
As the system bandwidth increases to support higher data rates and/or under certain operating conditions, a DS-CDMA system is more susceptible to frequency selective fading (i.e., different amounts of attenuation across the system bandwidth). For such a frequency-selective channel, time dispersion in the channel introduces inter-symbol interference (ISI), which can degrade system performance.
There is therefore a need in the art for a multiple-access CDMA-based system capable of mitigating ISI, supporting flexible operation, and providing improved system performance.